Postglacial spreadings of Plants. 209 
speak, the western conifers in their northwestward migration. FERNOW in ex- 
plaining why the Pacific coast forest of Alaska (consisting in the main of a 
mixture of the tideland, or Sitka spruce Picea sitchensis and coast hemlock 
Tsuga heterophylla, to which may be added, near timber line and farther west 
on the lower levels, the beautiful but useless alpine hemlock Tsauga Mertensiana) 
does not cover Kadiak Island and the Alaskan peninsula farther west, neglects 
the facts which the writer has advanced above to explain the distribution of 
the two distinct forests of Alaska. He adduces the following conditions which 
- doubtless have been strongly operative in checking the advance of the western 
and Pacific types of coniferous vegetation. To secure the extension of the 
coast forest, it is necessary that the winds should blow from the north and 
east from September to May, when the spruce and hemlock release their seed, 
and it should be dry in order to permit the cones to do so. The contrary 
usually happens: there is during these months a constant succession of south- 
east and south winds and the air is heavily charged with moisture‘). For 
this reason, the spread of the Pacific coast forests has been retarded, while 
the Atlantic coast forest of the interior shut off from such winds by high 
mountains including the McKinley range, had a decided advantage and has at 
last reached the Pacific coast ahead of the other forest. 
Spread of Plants from Southeastern Center. In eastern North 'America a 
large number of species came from the great forest in which broad-leaved 
trees and coniferous trees were intermixed the latter found especially perhaps 
during glacial times on the mountain tops, which had remained undisturbed 
in their original home. At the close of the long ice age these trees were in 
a plastic condition through the influence of the pressure of species and through 
the action of the physiographic vicissitudes to which these fictile forms were 
subjected: Many species, therefore, found in this remnant of the great Mio- 
cene forest (which covered eastern North America west of the coastal plain, 
east of the prairie region and south of a line stretching along the Ohio River 
northeastward to another line following the windings of the present west branch 
of the Susquehanna River to the Blue Ridge, thence along the Blue Ridge to 
the Schuylkill River, thence across to. the southeast side of Great Valley, thence 
to the Delaware River, which at this point marked the eastern extension of 
the forest) found congenial conditions in the most extensive land areas north 
of the terminal moraine where the species which had preceded in the first 
three floral waves formed open ecologic formations, which permitted, there- 
fore, new migrants to occupy the ground. An inspection of the forest maps 
to be found in the ninth volume of the Tenth Census Report, Forest Trees 
of North America, shows that the majority of trees, the geographic distribution 
of which is illustrated by the maps, proceeded from a territory which seems 
to center in the area of the present states of southern and central Pennsylvania, 
een West Virginia, Kentucky, Tennessee, western North Carolina, south- 
ı) FERNOW, B.E.: The Forests of Alaska. Forestry and Irrigation VII: 66-70. Feb. 1902. 
Harshberger, Survey N.-America, 14 
